1. Field of the Invention
The present invention relates to devices such as a copy machine, a printer and a facsimile, that can print on both sides of a sheet or that can turn a sheet upside down.
2. Discussion of the Background
A conventional reverse feeding device as shown in FIG. 7, has plural feeding rollers 101, feeding rollers 100, branch selector 104, reverse feeding rollers 102 and plural feeding rollers 103. The feeding rollers 101 transport a paper P to an area of the feeding roller 100 and the branch selector 104 while guiding the paper P along a guiding face 104a. Then, the feeding rollers 100 transport the paper P to a switch back path SB. The reverse feeding rollers 102 then transport the paper P to the feeding rollers 103 while guiding the paper P along a reverse guiding face 104b. 
FIG. 8 shows a top view of the feeding rollers 100 and the branch selector 104. Plural feeding rollers 100 are rotatably supported by an shaft 106. The branch selector 104 comprises a plurality of selector elements (nails) movably supported by a shaft 107 and located very near to the feeding rollers 100. Tips of the branch nails 104 overlap the feeding rollers 100, as shown in FIG. 8. FIG. 9(a), (b) are side views of the feeding rollers 100 and the branch nails 104. FIG. 9(a) shows a condition where there is no paper between the feeding rollers 100 and the branch nails 104. When the paper P is transported between the feeding rollers 100 and the branch nails 104, the tips of the branch nails 104 press on the paper P. On the other hand, the paper P is pressed toward the axis 106 by the branch nails 104. Accordingly, friction between the paper P and the feeding rollers 100 is increased and the paper P is transported reliably by the feeding rollers 100.
As shown in FIG. 8, guiding nails 105, which are similar to the branch nails 104, are also supported by the shaft 107. The guiding nails 105 are located farther than the branch nails 104 from the feeding rollers 100. Tips of the guiding nails 105 do not overlap the feeding rollers 100, as shown in FIG. 8. These guiding nails 105 do not have a function of pressing the paper P toward the feeding rollers 100, but instead have a function of guiding the paper P when the paper P is transported form the switch-back path SB to the feeding rollers 103.
Next we explain the reason why the guiding nails 105 do not overlap the feeding rollers. A recent model printer can print on a tab-paper Pt, as shown in FIG. 11. When using the tab-paper Pt, the tab-paper Pt is transported through the machine and through the reverse feeding device. When a printing area PP of the tab-paper Pt is transported between the branch nails 104 and the feeding rollers 100, the feeding power of the feeding rollers 100 is fully transferred to the tab-paper. However, after the printing area PP passes the branch nails 104 and the feeding rollers 100, transportation of the tab-paper may depend on the inertia of the tab-paper, since the tab area Ptab of the tab-paper Pt may not be caught by the nearest feeding roller 100. If the guiding nail 105xe2x80x2 overlaps the feeding rollers 100, as shown in FIG. 10, the tab area will be pressed by the guiding nail 105xe2x80x2, and this pressure will stop the transportation of the tab-paper Pt against its inertia. But since the tips of the guiding nails 105 are formed not to overlap the feeding rollers 100, as shown in FIG. 9(c), this does not occur.
However, as there are then gaps X between the guiding nails 105 and the feeding rollers 100 as shown in FIG. 9(c), when the paper P is transported by reverse feeding rollers 102, a front-end of the paper becomes embedded in the gaps X, and a paper jam occurs. This is especially a problem for a sheet after passing through a fixing device, as side edges of the paper are curved, and the risk of the paper jam increases.
Accordingly, one object of this invention is to provide a novel reverse feeding device which can feed a paper with stability.
It is another object of the invention to provide a reverse feeding device with a reduced risk of paper jamming.
According to a feature of the invention, the above and other objects are achieved by a reverse feeding apparatus for switching back and feeding a sheet, comprising at least one feeding roller positioned for feeding a sheet in a first direction from an upstream path to a downstream path; a reverse feeding roller located at the downstream path and positioned for feeding a sheet in a second direction that is different from the first direction; at least one branch nail movably mounted adjacent the feeding roller; at least one guiding nail movably mounted adjacent the feeding roller, wherein a tip of the branch nail and a tip of the guiding nail are positionable to overlap the feeding roller when the sheet is not being fed in the first direction by the feeding roller, a branch nail pressing device which presses the branch nail to the position overlapping the feeding roller to increase friction between a sheet being fed in the first direction and the feeding roller; and a guiding nail pressing device which presses the guiding nail to the position overlapping the feeding roller to permit a front end of a sheet being fed in the second direction to pass by a lower face of the guiding nail. The pressure applied by the guiding nail pressing device is smaller than the pressure applied by the branch nail pressing device.
According to this feature, the pressure applied by the guiding nail pressing device is set such that the inertial movement of a sheet moving in the upstream path is not obstructed by the guiding nail. Therefore, the guiding nail is less likely to cause paper jamming.
According to another feature of the invention, the above and other objects are achieved by a reverse feeding apparatus for switching back and feeding a sheet, comprising feeding roller means for feeding a sheet in a first direction from an upstream path to a downstream path; reverse feeding roller means for feeding a sheet in a second direction that is different from the first direction; at least one branch nail movably mounted adjacent the feeding roller means; at least one guiding nail movably mounted adjacent the feeding roller means, wherein a tip of the branch nail and a tip of the guiding nail are positionable to overlap the feeding roller means when the sheet is not being fed in the first direction by the feeding roller means, branch nail pressing means for pressing the branch nail to the position overlapping the feeding roller means to increase friction between a sheet being fed in the first direction and the feeding roller means; and guiding nail pressing means for pressing the guiding nail to the position overlapping the feeding roller means to permit a front end of a sheet being fed in the second direction to pass by a lower face of the guiding nail. The pressure applied by the guiding nail pressing means is smaller than the pressure applied by the branch nail pressing means.
According to this feature, the pressure applied by the guiding nail pressing means is set such that the inertial movement of a sheet moving in the upstream path is not obstructed by the guiding nail. Therefore, the guiding nail is less likely to cause paper jamming.
According to yet another feature of the invention, the above and other objects are achieved by a method for switching back and feeding a sheet, comprising the steps of using at least one feeding roller to feed a sheet in a first direction from an upstream path to a downstream path; using a reverse feeding roller located at the downstream path to feed the sheet in a second direction that is different from the first direction; using at least one branch nail movably mounted adjacent the feeding roller to press the paper sheet being fed in the first direction toward the at least one feeding roller; and using at least one guiding nail movably mounted adjacent the feeding roller to guide the sheet being fed in the second direction without obstructing the inertial movement of the sheet in the first direction.